You've got some great advice. I just want to add the following:
As already mentioned, over water is one of the safest places to fly. The "or something" might be that you read about the Vision Position System (VPS), if your craft even has it. See your manual, as it warns that it may not work properly over some surfaces, such as water. But that doesn't mean that you will lose control of your craft -- it just means that the VPS is not reliable at that point, so do not depend on it (my manual says "Operate the aircraft with great caution" in those situations).
Really, if you don't know that you need VPS (which is helpful in tight spots, such as indoors), then you don't even need to have it on.
Or you may have read something else entirely, but flying over water is safe when you have proper controller to aircraft signal (can see the camera signal on your controller app) and there are plenty of satellites acquired (there's a number next to the satellite icon on your controller app -- it's usually over a count of 10).
Chris